[lsr][term-fold] Restrict transform to low cost expansions (#74747)

This is a follow up to an item I noted in my submission comment for
e947f95. I don't have a real world example where this is triggering
unprofitably, but avoiding the transform when we estimate the loop to be
short running from profiling seems quite reasonable. It's also now come
up as a possibility in a regression twice in two days, so I'd like to
get this in to close out the possibility if nothing else.

The original review dropped the threshold for short trip count loops. I
will return to that in a separate review if this lands.

Author: preames

llvm/lib/Transforms/Scalar/LoopStrengthReduce.cpp

@@ -6762,7 +6762,7 @@ static llvm::PHINode *GetInductionVariable(const Loop &L, ScalarEvolution &SE,
 
 static std::optional<std::tuple<PHINode *, PHINode *, const SCEV *, bool>>
 canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
-                      const LoopInfo &LI) {
+                      const LoopInfo &LI, const TargetTransformInfo &TTI) {
   if (!L->isInnermost()) {
     LLVM_DEBUG(dbgs() << "Cannot fold on non-innermost loop\n");
     return std::nullopt;
@@ -6820,6 +6820,7 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
   PHINode *ToHelpFold = nullptr;
   const SCEV *TermValueS = nullptr;
   bool MustDropPoison = false;
+  auto InsertPt = L->getLoopPreheader()->getTerminator();
   for (PHINode &PN : L->getHeader()->phis()) {
     if (ToFold == &PN)
       continue;
@@ -6861,6 +6862,15 @@ canFoldTermCondOfLoop(Loop *L, ScalarEvolution &SE, DominatorTree &DT,
       continue;
     }
 
+    if (Expander.isHighCostExpansion(TermValueSLocal, L,
+                                     2*SCEVCheapExpansionBudget,
+                                     &TTI, InsertPt)) {
+      LLVM_DEBUG(
+          dbgs() << "Is too expensive to expand terminating value for phi node"
+                 << PN << "\n");
+      continue;
+    }
+
     // The candidate IV may have been otherwise dead and poison from the
     // very first iteration.  If we can't disprove that, we can't use the IV.
     if (!mustExecuteUBIfPoisonOnPathTo(&PN, LoopLatch->getTerminator(), &DT)) {
@@ -6986,7 +6996,7 @@ static bool ReduceLoopStrength(Loop *L, IVUsers &IU, ScalarEvolution &SE,
   }();
 
   if (EnableFormTerm) {
-    if (auto Opt = canFoldTermCondOfLoop(L, SE, DT, LI)) {
+    if (auto Opt = canFoldTermCondOfLoop(L, SE, DT, LI, TTI)) {
       auto [ToFold, ToHelpFold, TermValueS, MustDrop] = *Opt;
 
       Changed = true;

llvm/test/Transforms/LoopStrengthReduce/lsr-term-fold.ll

@@ -599,26 +599,15 @@ define void @expensive_expand_unknown_tc2(ptr %a, i32 %offset, i32 %n, i32 %step
 ; CHECK-NEXT:  entry:
 ; CHECK-NEXT:    [[OFFSET_NONZERO:%.*]] = or i32 [[OFFSET:%.*]], 1
 ; CHECK-NEXT:    [[UGLYGEP:%.*]] = getelementptr i8, ptr [[A:%.*]], i64 84
-; CHECK-NEXT:    [[SMAX:%.*]] = call i32 @llvm.smax.i32(i32 [[STEP:%.*]], i32 [[N:%.*]])
-; CHECK-NEXT:    [[TMP0:%.*]] = sub i32 [[SMAX]], [[STEP]]
-; CHECK-NEXT:    [[UMIN:%.*]] = call i32 @llvm.umin.i32(i32 [[TMP0]], i32 1)
-; CHECK-NEXT:    [[TMP1:%.*]] = sub i32 [[TMP0]], [[UMIN]]
-; CHECK-NEXT:    [[UMAX:%.*]] = call i32 @llvm.umax.i32(i32 [[STEP]], i32 1)
-; CHECK-NEXT:    [[TMP2:%.*]] = udiv i32 [[TMP1]], [[UMAX]]
-; CHECK-NEXT:    [[TMP3:%.*]] = add i32 [[UMIN]], [[TMP2]]
-; CHECK-NEXT:    [[TMP4:%.*]] = zext i32 [[TMP3]] to i64
-; CHECK-NEXT:    [[TMP5:%.*]] = add nuw nsw i64 [[TMP4]], 1
-; CHECK-NEXT:    [[TMP6:%.*]] = sext i32 [[OFFSET_NONZERO]] to i64
-; CHECK-NEXT:    [[TMP7:%.*]] = mul i64 [[TMP5]], [[TMP6]]
-; CHECK-NEXT:    [[TMP8:%.*]] = add nsw i64 [[TMP7]], 84
-; CHECK-NEXT:    [[SCEVGEP:%.*]] = getelementptr i8, ptr [[A]], i64 [[TMP8]]
 ; CHECK-NEXT:    br label [[FOR_BODY:%.*]]
 ; CHECK:       for.body:
 ; CHECK-NEXT:    [[LSR_IV1:%.*]] = phi ptr [ [[UGLYGEP2:%.*]], [[FOR_BODY]] ], [ [[UGLYGEP]], [[ENTRY:%.*]] ]
+; CHECK-NEXT:    [[LSR_IV:%.*]] = phi i32 [ [[LSR_IV_NEXT:%.*]], [[FOR_BODY]] ], [ 0, [[ENTRY]] ]
 ; CHECK-NEXT:    store i32 1, ptr [[LSR_IV1]], align 4
+; CHECK-NEXT:    [[LSR_IV_NEXT]] = add nsw i32 [[LSR_IV]], [[STEP:%.*]]
 ; CHECK-NEXT:    [[UGLYGEP2]] = getelementptr i8, ptr [[LSR_IV1]], i32 [[OFFSET_NONZERO]]
-; CHECK-NEXT:    [[LSR_FOLD_TERM_COND_REPLACED_TERM_COND:%.*]] = icmp eq ptr [[UGLYGEP2]], [[SCEVGEP]]
-; CHECK-NEXT:    br i1 [[LSR_FOLD_TERM_COND_REPLACED_TERM_COND]], label [[FOR_END:%.*]], label [[FOR_BODY]]
+; CHECK-NEXT:    [[EXITCOND_NOT:%.*]] = icmp sge i32 [[LSR_IV_NEXT]], [[N:%.*]]
+; CHECK-NEXT:    br i1 [[EXITCOND_NOT]], label [[FOR_END:%.*]], label [[FOR_BODY]]
 ; CHECK:       for.end:
 ; CHECK-NEXT:    ret void
 ;